Control circuits and methods for switching mode power supplies

ABSTRACT

Circuits and method for sensing a system output voltage change and transmitting an electrical signal to an auxiliary winding on the primary side. In an embodiment, a primary side controller detects the electrical signal and turns a power switch on and off to transfer energy from a primary winding to a secondary winding. A secondary side controller generates a voltage pulse at a terminal of the secondary winding when the system output voltage is below a reference voltage. The secondary side controller includes an input terminal connected to a system output, an output terminal connected to a terminal of the secondary winding, and a ground terminal connected to a ground potential of the system output.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is related to U.S. patent application Ser. No.12/248,665, filed Oct. 9, 2008, entitled “METHOD AND APPARATUS FORREDUCING STANDBY POWER OF WITCHING MODE POWER SUPPLIES” by inventorsYajiang Zhu et al., commonly assigned and incorporated by referenceherein for all purposes.

This application is also related to U.S. patent application Ser. No.12/136,760, filed Jun. 11, 2008, entitled “METHOD AND SYSTEM FOR PULSEFREQUENCY MODULATED SWITCHING MODE POWER SUPPLIES” by inventors YajiangZhu et al., commonly assigned and incorporated by reference herein forall purposes.

BACKGROUND OF THE INVENTION

The present invention is directed to switching mode power supplies(SMPS). More particularly, embodiments of the invention provide circuitsand methods of transferring system output status through thetransformer. Merely by way of example, the invention has been applied tocontrol the output voltage undershoot of a pulse width modulated (PWM)or a pulse frequency modulated (PFM) switching mode power supply forreducing the output voltage undershoot of switching mode power suppliesduring the load transient of the SMPS. But it would be recognized thatthe invention has a much broader range of applicability.

Switching mode power supplies (SMPS) have the advantages of smallersize, higher efficiency and larger output power capability, and arewidely applied in mobile phone chargers, notebook computer adapters andother fields. In recent years, green power supplies are emphasized,which require higher conversion efficiency and especially lower standbypower consumption. This leads to the continuous reduction of switchingfrequency in light load or no load conditions of an SMPS, because lowerswitching frequency in light load or no load conditions can reduceswitching loss of power transistor, thus increase power conversionefficiency and reduce system standby power. However, reducing switchingfrequency in an SMPS can also lead to many limitations. Some of theselimitations are described in more detail below.

In view of the foregoing, there is a need for an effective technique toreduce the output voltage drops during load transients or dynamic loadchanges of switching mode power supplies.

BRIEF SUMMARY OF THE INVENTION

In accordance with embodiments of the present invention, a variety oftechniques are described for transferring system output status through atransformer when the transformer is in a discontinuous conduction mode(DCM) and for reducing the output voltage undershoot during loadtransient or dynamic load change of switching mode power supplies.

In various embodiments, the present invention provides circuitimplementations for communicating system output changes to the primaryside through the transformer when the power supply system operates in adiscontinuous conduction mode (DCM). In DCM, after the primary windingstores energy during the turn-on period of the power switch and releasesthe energy to the secondary winding when the power switch is turned off,and there is a time interval that there are no direct currents in allthe windings of the transformer. In some embodiments, when the systemoutput voltage is lower than a predetermined value, electrical signalsare applied to a secondary winding of the transformer. These electricalsignals are communicated to a primary side controller through anauxiliary winding. The primary side controller senses the electricalsignals and turns on a power switch coupled in series with a primarywinding for a time period. The output voltage drop thus can be reducedduring load transients or dynamic load changes of switching mode powersupplies.

An embodiment of the present invention provides a switching mode powersupply (SMPS) which includes a transformer having a primary winding, asecondary winding for providing an output voltage of the SMPS, and anauxiliary winding. The SMPS also has a first control circuit, a secondcontrol circuit, and a power switch coupled to the primary winding. Thefirst control circuit is coupled to the secondary winding, and isconfigured to provide a first electrical signal to the secondary windingwhen the output voltage of the SMPS is less than a reference voltage,whereupon a second electrical signal is induced in the auxiliarywinding. The second control circuit is coupled to auxiliary winding andthe power switch. The second control circuit is configured to provide acontrol signal to the power switch in response to the second electricalsignal.

In an embodiment of the present invention, an integrated circuit (IC)for a secondary side of an SMPS includes a first terminal for couplingto a system output voltage, a second terminal for coupling to a groundpotential and a third terminal for coupling to a terminal of thesecondary winding of a switching mode power supply. When the systemoutput voltage is below a predetermined value, e.g., 4.7V for a 5Voutput voltage power supply, the secondary side IC applies a voltagepulse to the terminal of the secondary winding to generate avoltage/current ringing waveform during a discontinuous time of thetransformer. This ringing waveform produces an electrical signal at theauxiliary winding, which is then detected by a prime side controller IC.The primary side controller IC turns on the power switch of theswitching mode power supply for a time upon detection of the electricalsignal. This process continues until the system output voltage isgreater than a predefined value, such as 4.7V for a 5V output voltagepower supply.

According to an embodiment of the present invention, the primary sidecontroller IC includes a terminal FB for coupling to the auxiliarywinding and a terminal OUT for coupling to the power switch. The primaryside controller IC receives the electrical signal having a firstwaveform and a second waveform at terminal FB when the transformer is inthe discontinuous time and ignores the first pulse waveform, which maybe generated by parasitic components associated with the transformer,rather than the secondary side IC. When the primary side controlleridentifies that there are ringing signals after the first dampingringing waveforms through the terminal FB, it will turn on the powerswitch of the switching mode power supply for a time after it receivesthe ringing signal generated by the IC in the secondary side. Energy isdelivered from the primary side to the secondary side. This processcontinues until the system output voltage is larger than the specifiedvalue.

In another embodiment of the present invention, a 3-terminal integratedcircuit is provided to sense the system output voltage change and toapply a voltage/current pulse (or a series of pulses) to the secondarywinding for the generation of a resonant ringing waveform when thesystem output voltage is equal to or less than a predetermined value.The 3-terminal IC is powered by the system output voltage. The3-terminal IC includes an output terminal connected to a terminal of thesecondary winding and the anode of a rectifying diode in the secondaryside. The 3-terminal IC also includes a switch having one end forcoupling to the system output and another end for coupling to theterminal of the secondary winding. In an embodiment, the switch is aPMOS transistor having a source shorted to its body (an N-well) andcoupled to the system output voltage, a drain coupled to the anode ofthe rectifying diode, and a gate coupled to a control block, which isintegrated in the 3-terminal IC. When the voltage VCC of the 3-terminalIC is below a predetermined voltage, the control block turns on the PMOStransistor and causes a current flow in the secondary winding. Resonantringing voltage waveforms will be generated after the PMOS transistor isturned off.

In yet another embodiment of the present invention, a 3-terminalintegrated circuit is provided to sense the system output voltage changeand to apply a voltage pulse (or a series of pulses) to the secondarywinding for the generation of a resonant ringing waveform when thesystem output voltage is below or equal to a predetermined value. The3-terminal IC is powered by the system output voltage. The 3-terminal ICincludes an output terminal for coupling to a terminal of the secondarywinding and the cathode of the rectifying diode in the secondary side.The 3-terminal IC also includes a switch for coupling between the systemground potential and the cathode of the rectifying diode in thesecondary winding. In one embodiment, the switch is an NMOS transistorhaving a source shorted to its body (a P-well substrate) and coupled tothe ground potential of the system output, a drain coupled to thecathode of the rectifying diode, and a gate coupled to a control block,which is integrated in the 3-terminal IC. When the VCC output voltage ofthe 3-terminal IC is equal to or below a predetermined voltage value,the control block will turn on the NMOS transistor and induce a currentin the secondary winding. Resonant ringing voltage waveforms will begenerated after the NMOS device is turned off.

In still another embodiment of the present invention, the control blockof the 3-terminal IC includes resistors forming a voltage divider, acomparator, and a pulse generation circuit, which drives the gate of thePMOS or NMOS transistor.

In the embodiments of the present invention, the control IC in primaryside contains a resonant ringing waveform detector circuit configured toskip the first damping resonant ringing waveforms, detect the resonantringing waveforms generated by the secondary side control IC and turn onthe power switch for a specified time upon detection of the resonantringing waveforms.

In still another embodiment of the present invention, the secondary sideIC includes a second comparator for controlling the switch. The secondcomparator compares the anode voltage and cathode voltage of anintegrated diode and turn on the switch if the anode voltage is greaterthan the cathode voltage for the drain to substrate diode. In oneembodiment, the integrated diode is a part of the PMOS or NMOStransistor having the drain being the anode and the source and the bulksubstrate being the cathode. The transistor's on-resistance can be madesmall, whereby the voltage drop across the switch can be lower than 0.5Vduring the on time of the secondary winding. Then the externalrectifying diode can be eliminated, and the power supply conversionefficiency is increased because the switch will dissipate less powerthan the external rectifying diode, whose forward conduction voltage isabout 0.5V. In yet another embodiment, the second comparator is anoffset comparator.

An alternative embodiment of the present invention provides athree-terminal device for controlling a switching mode power supply(SMPS). The three-terminal device includes a first terminal for couplingto the output of the switching mode power supply, a second terminal forcoupling to a secondary side ground of the switching mode power supply,and a third terminal for coupling to a secondary winding. In adiscontinuous mode of the SMPS, the three-terminal device is configuredto provide an electric signal to a secondary winding in response to anoutput condition. In a conduction mode of the SMPS, the three-terminaldevice is configured to provide a low-voltage drop conduction path forthe current in the secondary winding.

In an embodiment of the above three-terminal device, an MOSFETtransistor has a drain for coupling to one terminal of the secondarywinding and a source for coupling to the ground terminal or the voltageoutput terminal of the SMPS. The three-terminal device also has acontrol unit configured to turn on and off the MOSFET.

Another embodiment of the invention provides a method of controlling theoutput power of a switching mode power supply (SMPS). The SMPS includesa transformer having a primary winding coupled to a power switch, asecondary winding for providing an output, and an auxiliary winding. Themethod includes detecting a change in the output at the secondarywinding, causing a current flow in the secondary winding, and inducing asignal in the auxiliary winding during a discontinuous time of the SMPS.The method also includes applying a control signal to the power switchin response to the induced signal in the auxiliary winding.

In an embodiment of the above method, detecting a change in the outputat the secondary winding includes comparing the output with a referencevalue. In another embodiment, causing a current flow in the secondarywinding includes transferring electric charges to the secondary windingupon detecting the change in the power supply output. In anotherembodiment, applying a control signal to the power switch to the inducedsignal in the auxiliary winding includes detecting a ringing waveform inthe induced signal in the auxiliary winding.

Various additional embodiments, features, and advantages of the presentinvention can be appreciated with reference to the detailed descriptionand accompanying drawings that follow.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example, and not by wayof limitation, in the figures of the accompanying drawings and in whichlike reference numerals refer to similar elements and in which:

FIG. 1 is a schematic diagram of a conventional primary side regulatedswitching mode power supply;

FIG. 2 shows the waveforms of an system output voltage at 50 Hz dynamicload changes of the power supply of FIG. 1;

FIG. 3 is a simplified system schematic diagram of an SMPS having arectifying diode located in the upper side of the secondary winding, inaccordance with an embodiment of the present invention;

FIG. 4 is a simplified system schematic diagram of an SMPS having arectifying diode located in the lower side of the secondary winding, inaccordance with an embodiment of the present invention;

FIG. 5A is a simplified block diagram illustrating a control IC for asecondary side of a SMPS according to an embodiment of the presentinvention;

FIG. 5B is a simplified circuit diagram of a comparator having built-inhysteresis, in accordance with an embodiment of the present invention;

FIG. 6 is a simplified block diagram illustrating a control IC in thesecondary side according to another embodiment of the present invention;

FIG. 7 shows the waveforms of the conventional SMPS of FIG. 1, where thediscontinuous time Tdis is large, resulting in a Vout drop duringdynamic load changes;

FIG. 8 shows the waveforms of the SMPS according to embodiments of thepresent invention, where the discontinuous time Tdis is reduced,resulting in a much smaller Vout drop during dynamic load changes; and

FIG. 9 is a schematic diagram of an embodiment of a secondary sidecontrol IC having a MOSFET switch replacing the external rectifyingdiode in the secondary winding, in accordance with an embodiment of thepresent invention;

DETAILED DESCRIPTION OF THE INVENTION

As described above, new energy efficiency requirements have led tocontinued reduction of switching frequency in SMPS. But slower switchingfrequency in light load or no load conditions results in a slowerresponse to system output load transients from light loads to heavyloads, which can lead to a large system output voltage undershoot duringa single load transient, or periodic load transients between no load tofull load. As used herein, the term “dynamic load changes” refers torapidly changes in output voltage and/or current at the system output.

Restrain output voltage spike during load transient or dynamic loadchange within a specific range is often desired in an SMPS. For example,some specifications of mobile phones require that the charger outputvoltage must be higher than 4.2V when the output current varies within arange from 0 mA to 700 mA. Some notebook computers or LCD TVs may alsorequire that adapters have output voltages that vary less than +/−10percent of their nominal operating voltage when switching from asleeping mode to a normal operating mode, and vice versa.

For pulse frequency modulated (PFM) systems, the switching frequencydecreases when the load current decreases, and the switching frequencycan be about a few hundred Hz in no load conditions for a 30 mW standbypower mobile phone charger. For pulse width modulated (PWM) systems, thesystems operate in a burst mode under no load conditions, and the PWMcontrollers do not generate switching signals for tens of millisecondsafter outputting a burst of pulses having a minimum pulse width. In bothsystems, the controllers need a few or tens of milliseconds to increasethe switching frequency from a few hundred Hz to tens of kilo Hz (PFM)or change from the burst mode to a maximum duty cycle (PWM).

FIG. 1 is a simplified block diagram of a conventional switching modepower supply (SMPS) 100. As shown in FIG. 1, SMPS 100 includes acontroller 101 with a power supply terminal VCC, a feedback terminal FB,an output terminal OUT, and a current sense terminal CS. Controller 101can be a PWM controller or a PFM controller. SMPS 100 also includes atransformer that has a primary winding 105, a secondary winding 106 andan auxiliary winding 107. SMPS 100 also includes rectifying diodes 102that rectify an AC mains voltage source to generate a rectified DC inputvoltage Vin. The output of the power supply Vout is provided bysecondary winding 106 and a rectifying circuit including diode 113 andcapacitor 114. The rectified DC input voltage Vin supplies power tocontroller 101 at the power supply terminal VCC through a resistor 104and a bypass capacitor 111 at start up. While in the normal operationmode, auxiliary winging 107 supplies power to controller 101 via arectifier diode 110. Current sense terminal CS is coupled to a terminalof a bipolar transistor 108 to sense a primary current flowing throughprimary winding 105 and bipolar transistor 108. Controller 101 deliversa control pulse signal at output terminal OUT to turn on and off bipolartransistor 108. Voltages are induced in the auxiliary and secondarywindings when transistor 108 is turned off. Resistors 112 and 113 form avoltage divider to reduce the induced voltage at auxiliary winding. Thereduced voltage is fed back to controller 101 at input terminal FB forfurther processing.

The output voltage Vout is regulated by a control block in controller101, which receives a current signal from a current sense resistor 109and a voltage signal from resistors 112 and 113. The average value ofVout can thus be controlled by the negative feedback loop formed by thecontroller 101, power switch 108 and the transformer. However, thetransient and dynamic performance of Vout is limited by the bandwidth ofthe feedback loop in light load conditions, which is in the range of afew hundred Hz. This leads to poor dynamic load response of Vout, asillustrated in FIG. 2.

FIG. 2 shows the waveforms of an system output voltage at 50 Hz dynamicload changes of the power supply of FIG. 1, and illustrates that thelong ramp-up time can result in output voltage drops of several voltsduring a sudden output load change. As shown, the DC output voltage Voutof the secondary winding is about 5.46V at light load conditions, andcan drop to about 1.42V as the load increases suddenly.

The voltage drop of the secondary winding is fed back to the primaryside controller 101 through auxiliary winding 107 and resistors 112 and113. Controller 101 then turns on and off transistor 108 to transferenergy from primary winding 105 to secondary winding 106, whereby theoutput voltage Vout maintains a stable voltage level below 5.46V at agiven load. Output voltage Vout reaches again 5.46V when light loadconditions return.

Embodiments of the present invention provide methods for enhancing thedynamic performance of the power supplies. In some embodiments, themethod includes monitoring the output voltage status and instructing thecontroller in primary side to take actions when Vout is below aspecified value.

Some embodiments of the invention provide a method of controlling theoutput power of a switching mode power supply (SMPS), the SMPS includinga transformer having a primary winding coupled to a power switch, asecondary winding for providing an output, and an auxiliary winding. Themethod includes detecting a change in the output at the secondarywinding and causing a current flow in the secondary winding, whichinduces a signal in the auxiliary winding during a discontinuous time ofthe SMPS. The method also includes applying a control signal to thepower switch in response to the induced signal in the auxiliary winding.Further details of the embodiments are provided in the examplesdescribed below.

FIG. 3 is a simplified system schematic diagram of a SMPS having arectifying diode located in the upper side of the secondary winding, inaccordance with an embodiment of the present invention; FIG. 3 showsSMPS 300 configured in a flyback converter topology. System 300 includesa primary winding 305 coupled in series to a power transistor 308, asecondary winding 306, and an auxiliary winding 307. A primary sidecontrol circuit 301 receives a voltage signal through a FB inputterminal and a current sense signal through a CS input terminal. Controlcircuit 301 turns on and off power transistor 308 based on the voltageand/or current signals. When power transistor 308 is turned on, aprimary current Ip builds in primary winding 305, which stores energy.The energy stored in primary winding 305 is transferred to secondarywinding 306 during the turn-off time interval of power transistor 308. Arectifier element 313 and a smoothing capacitor 314 in secondary winding306 converts a secondary voltage Vs into a DC system voltage Vout tosupply to a load 315. System output voltage Vout is monitored by asecondary side controller circuit 316. Controller circuit 316 includesan input terminal VCC, an output terminal Vo, and a GND terminal, whichis connected to the ground potential of the system output Vout. TerminalVCC is coupled to output voltage Vout, and terminal Vo is coupled to oneend of the secondary winding. System 300 can operate in a discontinuousconduction mode (DCM) in no load or light load conditions, i.e., theenergy in primary winding 305 and secondary winding 306 can drop tozero. These are time intervals during which there are no direct currentsin the primary winding, the secondary winding and the auxiliary windingof the transformer. These time intervals can be used to transfer Voutchange information without additional system overhead. The changeinformation of output voltage Vout is sent by control circuit 316 in thesecondary side, and received by control circuit 301 in the primary side.

In an embodiment, secondary side control circuit 316 includes a controlcircuit 320 and a switch. Control circuit 320 turns on the switch whensystem output voltage Vout is below a predetermined value. Furtherdetails of control circuit 316 is described below in connection withFIGS. 5A and 5B.

Primary side control circuit 301 includes a dynamic event detectioncircuit 330 connected between the FB terminal and a driver block 334.Dynamic event detection circuit 330 is connected in parallel with aconventional voltage control loop 332 and provides the ability toprimary control circuit 301 to respond to rapid changes in loadvariations.

FIG. 4 is a simplified system schematic diagram of an SMPS having arectifying diode located in the lower side of the secondary winding, inaccordance with an embodiment of the present invention. As shown, arectifier element 413 is connected in the lower side of the secondarywinding. In this case, an output pin Vo of a secondary side controlcircuit 416 is also connected to the lower side of the secondarywinding. Primary side control circuit 401 is the same as control circuit301 of FIG. 3. Secondary side control circuit 416 will be described indetail in following sections.

FIG. 5A is an exemplary circuit diagram of secondary side controllercircuit 316 of FIG. 3 in accordance with one embodiment of the presentinvention. A change information transfer block 500 includes a terminalVCC for coupling to output voltage Vout and a terminal Vo for couplingto an end of secondary winding 306. Voltage Vout (at terminal VCC inFIG. 5A) is coupled to a voltage divider formed by resistors 501 and502. Attenuated voltage 510 is applied to a terminal of a comparator503. Comparator 503 compares voltage 510 with an internal referencevoltage Vref to sense a change at system output voltage Vout. If theattenuated Vout is below the reference voltage Vref, comparator 503 willoutput a high logic state, which triggers a pulse generator 504 togenerate a pulse or a series of pulses 505. Pulse(s) 505 then turns on aswitch 506. Switch 506, when turned on, will transfer a portion of theelectric charge stored at capacitor 314 to secondary winding 306 (FIG.3). The transferred electric charge causes a current flow in secondarywinding 306. Hence, any dynamic change at output voltage Vout can becommunicated to the primary side while the transformer is in adiscontinuous mode (DCM) state. When switch 506 is turned off, thecurrent at secondary winding 306 will go back to zero; an oscillation orresonant ringing waveform at a frequency determined by the parasiticinductance and capacitance will occur in secondary winding 306, andresonant ringing waveform having similar shapes also appears inauxiliary winding 307. Primary side controller 301 (FIG. 3) detects thisringing waveform and turns on and off power switch 308 to transferenergy from primary winding 305 to secondary winding 306. In oneembodiment of the present invention, switch 506 is a PMOS transistor.

In an embodiment, comparator 503 can be a comparator having ahysteresis. The hysteresis can prevent noise signals occurring at VCCfrom triggering a change of logic states at the comparator output. Asshown in FIG. 5B, comparator 503 may include a hysteresis generationcircuit to form a hysteresis of +/−Vh centered at Vref′.

FIG. 6 is a simplified block diagram illustrating a control IC in thesecondary side according to another embodiment of the present invention.FIG. 6 shows an exemplary implementation of secondary side controllercircuit 416 of FIG. 4 in accordance with another embodiment of thepresent invention. Change information transfer block 600 includes aterminal VCC for coupling to smoothing capacitor 414, a terminal Vo forcoupling to a terminal of secondary winding 406, and a ground terminalfor coupling to a ground potential of the secondary side voltage output.Change information transfer block 600 includes a voltage divider formedby resistors 601 and 602 to attenuate voltage Vout that is applied atterminal VCC. A comparator 603 compares an attenuated voltage 610 withan internal reference voltage Vref and generate a control signal whenthe attenuated voltage is equal to or less than the reference voltage. Apulse generator 604 generates at least one pulse when triggered by thecontrol signal. Pulse generator 604 is similar to pulse generator 504 ofFIG. 5, and both can be a one-shot pulse generator or a multivibratoroscillator, etc. A switch 606 is coupled between the ground terminal andterminal Vo. The pulse turns on and off switch 606 that, when turned on,transfer a portion of the electric charge stored in capacitor 414 tosecondary winding 406. In one embodiment of the present invention,switch 606 is an NMOS transistor having source and substrate connectedto the ground potential of output voltage Vout.

Further details of the operation of a conventional SMPS and embodimentsof the present invention are described with reference to the waveformdiagrams in FIGS. 7 and 8.

FIG. 7 illustrates exemplary simplified voltage and current waveforms ofa conventional SMPS of FIG. 1. OUT 705 is the switching signal of theprimary side controller 101 for turning on transistor 108. Whiletransistor 108 is turned on (corresponding to time interval Tonp),primary side current Ip 704 ramps up linearly from 0 to its maximumvalue, and primary winding 105 stores magnetic energy. Secondary sidewinding voltage Vs is negative and diode 113 is reversed biased whiletransistor 108 is turned on. When transistor 108 is turned off at timet1, primary winding 105 releases the stored energy to secondary winding106, Vs goes to a positive peak value (with some overshoots), secondarycurrent Is 703 charges capacitor 114 and goes to zero at time t2. At t2,a damped ringing waveform occurs at Vs with a resonant frequencydetermined by the parasitic inductance and capacitance of the secondarywinding and lasts till t3. The SMPS operates in the discontinuousconduction mode (DCM) at the time interval between t2 and t4, wherethere are no energy stored in primary and secondary windings. Dependingon the load conditions, Vout starts drooping while the SMPS is in theDCM state. Power switch 108 will be turned on again by controller 101after a discontinuous time Tdis based on the feedback loop. In lightload or no load conditions, Tdis can be in the range of about a few toabout tens of milliseconds. If load transient or load dynamic occursduring the long Tdis time, especially after the non-conduction ofrectifying diode 113 (FIG. 1) such as at instant t2 in FIG. 7, outputvoltage Vout may droop for milliseconds, resulting in a Vout drop (to1.42V), as also illustrated in FIG. 2.

The Vout drop can be significantly reduced according to an embodiment ofthe present invention. FIG. 8 shows exemplary simplified voltage andcurrent waveforms of switch control signal and current of primarywinding, output signal of secondary side control circuit, and currentand voltage of secondary winding, with reference to SMPS 300 of FIG. 3.Out 806 is the switching signal of transistor 308. Ip 805 is the primaryswitch current of primary winding 305. Voltage Vo 803 is the voltagepulse at output terminal Vo of secondary side control circuit 316 (FIGS.3 and 5), which is generated when Vout (corresponds to VCC in FIG. 8) isbelow a reference voltage (e.g., 4.7V for a 5V output system). Is 804 isthe current flowing across diode 313 during the conduction time Tons(between t1 and t2) of secondary winding 306. Is 802 is the currentpulse flowing in the secondary winding while voltage pulse 803 ispresent (between t3 and t4). When current pulse Is 802 goes to zero att4, a ringing waveform 801 is generated in the secondary and auxiliarywindings.

Ringing waveform 801 in the secondary winding is fed back to the FBinput of primary side controller 301 through the auxiliary winding anddetected by dynamic event detection circuit 330 (FIG. 3). In anembodiment, detection circuit 330 receives the electrical signal havinga first waveform and a second waveform at terminal FB when thetransformer is in the discontinuous time and is configured to ignore thefirst pulse waveform, which may be generated by the parasitic componentsassociated with the transformer, rather than the secondary side IC.Detection circuit 330 can be implemented, for example, usingconventional circuit design techniques. Upon the detection of a dynamicchange event at output voltage Vout, control circuit 301 turns on powerswitch 308 to store and transfer energy from the primary side to thesecondary side, as indicated by the successive Ip waveforms 805-1,804-1, 805-2, and 804-2 after the detection of the ringing waveform.Vout (equals to VCC in FIG. 8) begins to increase, and thus results in aVout voltage drop to about 4.5V, compared with Vout dropping to about1.4V in the conventional system of FIG. 1 and FIG. 2.

The secondary side control circuit can also include active rectificationin the secondary winding according to an embodiment of the presentinvention. FIG. 9 is a simplified schematic diagram of an integratedcircuit 900 that can replace control circuit 416 including diode 413(FIG. 4). Integrated circuit 900 includes an input terminal VCC forcoupling to output voltage Vout, an output terminal Vo for coupling to aterminal of second winding, and a ground terminal for coupling to aground potential of output voltage Vout. Integrated circuit 900 furtherincludes a MOSFET transistor having a drain coupled to output terminalVo and a source coupled to the ground terminal. In addition, Integratedcircuit 900 includes a control unit that turns the MOSFET transistor onand off.

In one embodiment, the control unit includes a first control circuit 950to turn on the MOSFET for a time period when output voltage Vout isequal to or less than an internal reference voltage. The control unitalso includes a second control circuit 970 to turn on the MOSFET duringa conduction time of the second winding. In one embodiment, theconduction time is determined by a current Is flowing through secondarywinding 406. The control unit further includes a logic gate 905 thatwires outputs 922 and 926 of the respective first and second controlcircuits into a single control signal 930 for switching the MOSFETtransistor.

In one embodiment of the present invention, first control circuit 950includes a voltage divider formed by resistors 901 and 902 to attenuatevoltage Vout (at terminal VCC). A comparator 903 compares the attenuatedvoltage with the internal reference voltage to produce a comparisonresult 920. In one embodiment, comparison result 920 is at logic highwhen the attenuated voltage is equal to or less than the referencevoltage. A pulse generator 904 generates at least one pulse 922 when thecomparator is at logic high. Pulse 922 turns on the MOSFET via logicgate 905.

In one embodiment of the present invention, the second control circuit970 includes a comparator 908 having a positive input coupled to thesource or the bulk substrate of the MOSFET and a negative input coupledto the drain of the MOSFET through an offset voltage source 907. Offsetvoltage source 907 ensures that comparator 908 generates a turnoffsignal during the discontinuous time of the transformer. A diode 909 isconnected across the source and drain of the MOSFET with the anodecoupled to the source and the cathode coupled to the drain. Upon theturning off of power switch 408 in the primary side, the anode voltageof diode 909 will be higher than its cathode voltage. The output ofcomparator 908 will be high to turn on switch 906 via logic gate 905. Ifthe size of MOSFET 906 is sufficiently large, its on-resistance will besmall, the maximum voltage drop across the switch can be smaller than0.5V during the on time of the secondary winding.

In one embodiment, the discrete rectifying diode 413 in FIG. 4 can beeliminated, and power supply conversion efficiency is increased due toless power dissipation on the MOSFET transistor compared with discreterectifying diode 413, whose forward conduction voltage is around 0.5V.

In one embodiment, the offset voltage source is programmable since itseffect is to raise or lower the voltage level at which comparator 908will switch from one state to the other.

The above description is illustrative and not restrictive. Manyvariations of the invention will become apparent to those skill in theart upon review of this disclosure. The scope of the invention should,therefore, not be limited the above description.

1. A switching mode power supply (SMPS), comprising: a transformerhaving: a primary winding; a secondary winding for providing an outputvoltage of the SMPS; and an auxiliary winding; a power switch coupled tothe primary winding; a first control circuit coupled to the secondarywinding, the first control circuit being configured to provide a firstelectrical signal to the secondary winding when the output voltage ofthe SMPS is less than a reference voltage, whereupon a second electricalsignal is induced in the auxiliary winding; and a second control circuitcoupled to auxiliary winding and the power switch, the second controlcircuit being configured to provide a control signal to the power switchin response to the second electrical signal.
 2. The switching mode powersupply of claim 1 wherein the first control circuit comprises: a firstterminal coupled to an output capacitor to receive the output voltage ofthe SMPS; and a second terminal coupled to secondary winding; whereinthe first control circuit is configured to transfer electric chargesfrom the output capacitor to the second winding when the output voltageis equal to or less than a reference voltage.
 3. The switching modepower supply of claim 1 wherein the first control circuit comprises: avoltage divider coupled to the output voltage and configured togenerated an attenuated voltage; a first comparator having: a firstinput terminal coupled to the attenuated voltage; a second inputterminal coupled to the reference voltage; and an output terminal; apulse generator coupled to the comparator; and a switch coupled to thepulse generator; wherein the pulse generator is configured to turn onthe switch when the attenuated voltage is less than the referencevoltage.
 4. The switching mode power supply of claim 3 wherein theswitch is a PMOS or an NMOS transistor.
 5. The switching mode powersupply of claim 3 wherein the switch is connected between a terminal ofthe secondary winding and an output terminal of the SMPS.
 6. Theswitching mode power supply of claim 1 wherein the secondary winding isin a discontinuous state substantially free from current flow, when theoutput voltage of the SMPS is less than a reference voltage.
 7. Theswitching mode power supply of claim 1 wherein the second controlcircuit comprises: a feedback terminal coupled to the auxiliary windingthrough a coupling network, the feedback terminal being configured toreceive a feedback signal from the coupling network; a driver unitconfigured to drive the power switch; a sense current input terminalcoupled to a terminal of the power switch; a control unit coupled to thesense current input terminal and the feedback terminal; and a dynamicevent detection circuit coupled to the feedback terminal and configuredto detect the electrical signal.
 8. The switching mode power supply ofclaim 7 wherein the coupling network is a voltage divider network. 9.The switching mode power supply of claim 7 wherein the dynamic eventdetection circuit is configured to activate the driver unit for turningon the power switch for a predetermined time period upon detecting theelectrical signal.
 10. The switching mode power supply of claim 9wherein the dynamic event detection circuit is configured to detect aringing waveform in the auxiliary winding.
 11. A three-terminal devicefor controlling a switching mode power supply (SMPS), comprising a firstterminal for coupling to the output of the switching mode power supply;a second terminal for coupling to a secondary side ground of theswitching mode power supply; a third terminal for coupling to asecondary winding; wherein, in a discontinuous mode of the SMPS, thethree-terminal device is configured to provide an electric signal to asecondary winding in response to an output condition; wherein, in aconduction mode of the SMPS, the three-terminal device is configured toprovide a conduction path for the current in the secondary winding. 12.The three-terminal device of claim 11, further comprising: a MOSFETtransistor having a drain for coupling to one terminal of the secondarywinding and a source for coupling to the ground terminal or the voltageoutput terminal of the SMPS; and a control unit configured to turn onand off the MOSFET.
 13. The three-terminal device of claim 12 whereinthe MOSFET is a PMOS transistor having a bulk substrate coupled to thesource; wherein the drain acts as a cathode of a rectifying element andthe bulk substrate or source acts as an anode of the rectifying element.14. The three-terminal device of claim 12 wherein the control unitcomprises: a first control circuit configured to turn on the MOSFETtransistor for a time period when the output voltage is equal to or lessthan a reference voltage; a second control circuit configured to turn onthe MOSFET transistor during a conduction time of the second winding;and a logic gate having a first input coupled to a first output of thefirst control circuit and a second input coupled to a second output ofthe second control circuit, and a first output coupled to a gate of theMOSFET transistor.
 15. The three-terminal device of claim 14 wherein thefirst control circuit comprises: a voltage divider coupled to the inputterminal and configured to produce an attenuated voltage; a firstcomparator having a third input coupled to the attenuated voltage, afourth input coupled to the reference voltage, and a third output; and apulse generator coupled to the third output and configured to generateat least one pulse based on a comparison result of the first comparator;wherein the at least one pulse turns on the MOSFET via the logic gate.16. The three-terminal device of claim 14 wherein the second controlcircuit comprises: an offset voltage source; and a second comparatorhaving a fifth input coupled to the drain of the MOSFET via the offsetvoltage source and a sixth input coupled to the source or bulk substrateof the MOSFET; wherein the second comparator generates a control signalto turn on the MOSFET via the logic gate.
 17. A method of controllingthe output power of a switching mode power supply (SMPS), the SMPSincluding a transformer having a primary winding coupled to a powerswitch, a secondary winding for providing an output, and an auxiliarywinding, the method comprising: detecting a change in the output at thesecondary winding; causing a current flow in the secondary winding,which induces a signal in the auxiliary winding during a discontinuoustime of the SMPS; and applying a control signal to the power switch inresponse to the induced signal in the auxiliary winding.
 18. The methodof claim 17, wherein detecting a change in the output at the secondarywinding comprises comparing the output with a reference value.
 19. Themethod of claim 17, wherein causing a current flow in the secondarywinding comprises transferring electric charges to the secondary windingupon detecting the change.
 20. The method of claim 17, wherein turningon the power device for a time interval in response to the inducedsignal in the auxiliary winding comprises detecting a ringing waveformin the induced signal in the auxiliary winding.